1. Introduction
This invention relates to an improved method of treating the surface of an organic polymer substrate prior to electroless deposition of a metal coating. The method is easy to use, represents a reduction in the number of pretreatment steps necessary prior to electroless deposition of a metal onto the substrate and provides improved adhesion between the substrate and metal deposit.
2. Discussion of Prior Art
Even though significant progress has been made in the art of plating metals on plastics, the adhesion between the coating and the plastic still leaves much to be desired. Poor adhesion between the plastic and the metal plate allows differential dimensional changes with temperature which may result in warping, blistering, and cracking of the metallized product. Consequently, strong adhesion between a plastic substrate and the plated metallic layer is essential for any application in which the product is subjected to significant temperature fluctuations.
Numerous methods are proposed in the prior art for depositing a metal coating over plastic. The method most commonly used involves steps including surface converting a plastic part with an oxidant such as a solution of alkaline permanganate or sulfuric acid containing a source of hexavalent chromium ions, deposition of a conductive, adherent metallic film by chemical reduction followed by electrodeposition of an intermediate layer, frequently copper, and finally a layer of a desired outer metal coating such as chromium, nickel, gold, solder, silver or zinc. Only moderate bond strength between a plastic substrate and metal coating is obtained by this method. Also relatively high temperatures are required for the surface converting step and careful control of the chromium ion concentration is necessary.
Plastics show a relatively poor affinity for metal and to promote a stronger bond between a plastic substrate and a metallic coating the prior art has frequently resorted to roughening the plastic surface to provide locking or keying between the surface and its coating. The surface of a molded plastic article normally is glossy and quite hydrophobic. Consequently, this surface is unreceptive to aqueous solutions used in electroless metal deposition. Since the sensitizing and activating solutions will not wet the surface, the metal ions are not adsorbed onto the surface and deposition of the metal cannot proceed.
Rendering the surface of the substrate hydrophilic by roughening has been common practice in plating plastic materials. Initially, this surface roughening was accomplished by some form of mechanical deglazing, such as scrubbing with an abrasive slurry, wet tumbling, dry rolling or abrasive (sand) blasting. These procedures generally lead to a composite having an adequate bond between substrate and coating, but due to relatively large visible irregularities on the plastic surface formed during the roughening operation, a thick metal coating must be applied to avoid surface defects and obtain a coating having a smooth, highly polished appearance. Mechanical deglazing of the surface has been found to be fairly effective, but is extremely costly in that many parts have to be finished by hand. Another disadvantage to mechanical etching is that it is hard to control and many problems are encountered when the surface abrasion is carried too far. Methods of mechanical roughening are not applicable to three-dimensional products or more particularly, printed circuit boards having through-holes. In any event, adhesion values above a few pounds per inch are only rarely obtainable.
Roughening has also been accomplished chemically using an acidic etch solution or a solvent for dissolving a portion of the plastic surface. Chemical deglazing or etching techniques usually require use of strong, acidic solutions such as sulfuric acid and chromate salts. The latter treatment was found to have the effect of activating bonding sites for subsequent electroless metal deposition. Chemical etching by an acid chromate oxidizing solution was then found to be more effective when the surface was pretreated with a strong caustic, such as sodium hydroxide, combined with immersion in a reducing solution containing hydrochloric acid after the etching.
Further improvements of bond adhesion included using a pretreatment emulsion as described in U.S. Pat. No. 3,574,070. The emulsion consisted of a non-solvent for the plastic and a solvent for the plastic which were emulsifiable.
U.S. Pat. No. 4,594,311 describes a process for restoring hydrophobicity to the exposed surface of an adhesion promoted resinous surface of a base material after a catalytic resistless image has been imposed thereon. The process involves exposure of the treated resinous surface to a solvent or solvent vapors in order to restore hydrophobicity to the surface.
Pretreatment of an ABS polymer substrate prior to electroless deposition of metal is described in a 1969 U.S. Pat. No. 3,445,350. The method relates to specific solvents which may be used with the specific ABS resins to achieve improved adhesion.
Further studies in the area of adhesion promotion of ABS resins, including platable grades, showed that even though bond strength was improved, organic pre-conditioning has been observed to have an adverse effect on surface appearance. It was observed that when an ABS resin is organically pre-conditioned there is incomplete drainage of the pre-conditioning solution from the articles when they are withdrawn from the tank. Such problems showed themselves as defects in the article surface after electroless plating.
Another attempt at improvement was made and disclosed in U.S. Pat. No. 3,689,303. This process for pretreatment added an additional step of contacting an ABS resin article with an oxidizing agent prior to the organic pre-conditioning step. The method involves an 8-10 step process for pretreating an ABS resinous substrate prior to electroless deposition of metal.
Another patent, U.S. Pat. No. 4,552,626 teaches a combined pretreatment step of deglazing and removal of filler by use of an acid bath for polyamides. This step is preferred over the use of an organic solvent bath to deglaze the substrate, which may result in gelling of the resin if the solvent is not dilute.
U.S. Pat. No. 3,963,590 describes a process for electroplating polyoxymethylene (polyacetal). The pretreatment process involves treatment with quinoline or gamma-butyrolactone prior to acid etching.
Use of such techniques may result in degradation of the molecules forming the surface of the substrate, and may decrease both tensile and impact strength of the substrate due to swelling and cracking of the entire substrate material. Several of the prior art solvents used for organic pre-conditioning are also not suitable because they are known to remove significant amounts of the resin or resin filler, thus having a direct effect on the integrity of line and space dimensions of the substrate during further processing. For example, excessive roughening of a plastic substrate can hinder subsequent processing steps such as metallization, electrodeposition of photoresist materials, imaging and etching. If imaging of a deposited photoresist is desired, roughness may prevent sharp line definition of the photoresist materials. Also, a subsequently applied metal film will be difficult to remove due to plating in deep crevices on the surface of the plastic substrate.
While such methods as described above increase adhesion, they are often not transferable to all substrates and particularly not to all grades of engineering thermoplastic substrates. Numerous prior art patents confirm the lack of transferability of pretreatment processes from one substrate to the next or to substrates containing various fillers.
U.S. Pat. No. 3,567,594 discloses a process for treating of plastic substrates with acid chromate etch followed by an HF treatment prior to metallization. The process was disclosed as being applicable to certain ethylene propylene copolymers and polypropylene and was represented as being utile for polyesters. Donovan III, et al., in U.S. Pat. Nos. 4,325,991 and 325,992, evaluated the process of U.S. Pat. No. 3,567,594 on a platable grade of propylene polymer and found it to be functional. However, when they applied the same precess to a mineral-filled polyester substrate, the procedure failed. This lead to their disclosed methods, in the above referenced patents, for electroless plating of polyester substrates including pretreatment steps of contacting the surface with a detergent and hydrolizer prior to conditioning with an alkaline solution and etching with an acid fluoride solution.
A process disclosed as being applicable to polyimide substrates is disclosed in U.S. Pat. No. 4,775,449. The disclosed process provides for improving adhesion of metal to a polyimide surface without physical modification or degradation of the surface structure. The process involves pretreatment of the polyimide surface with an adhesion promoting compound containing a nitrogen-oxygen moiety prior to plating.
U.S. patent application Ser. No. 07/416,535, filed Oct. 3, 1989 now U.S. Pat. No. 5,178,956, and subject to common assignment with the present invention (hereafter the "Copending Application"), discloses a pretreatment process for polyetherimide or polyamides substrates. The process uses an organic lactone conditioner immediately followed by an aqueous rinse step to promote adhesion.
When organic solvents are used as conditioners, the solvent acts to swell the surface of the substrate. This swelling, generally seen as a white film on the substrate, aids in subsequent processing prior to electroless plating but is detrimental if not entirely removed. Any residue remaining on the surface or in the pores will result in areas of low adhesion. Use of organic solvents requires operation in very narrow process windows for selection of temperature of the conditioner and dwell time in the conditioner. Operation outside of these narrow process windows may result in loss of flexural strength of the substrate.
At minimal immersion times in the organic lactone conditioner of the Copending Application, it was found that after subsequent processing of a polyetherimide substrate, Ultem.TM. 2312 (manufactured by General Electric Company), there was a loss of 12 to 20% of the initial flexural strength as measured by ASTM Method 790. However, this loss of flexural strength does not apply to all grades of Ultem.TM. resins. For example, Ultem.TM. 3452 exhibits no change in flexural properties after treatment by the process of the above referenced application.
Aqueous solutions of metal hydroxides have been used in the prior art to etch polyimide substrates. U.S. Pat. No. 4,857,143 discloses an etchant suitable for use with fully or partially cured polyimides. The etchant is a metal hydroxide solution in combination with a metal carbonate, metal sulfate or metal phosphate. Etchants are known to attack plastics and are therefore not suitable for use as conditioners. Conditioners work to alter the chemistry of the plastic surface without extensive roughening of the surface.
U.S. Pat. No. 3,770,528 discloses a method for treating polyimides during the manufacture of composite laminated articles. The treatment consists essentially in subjecting the surface of a thin film of polyimide material to a solution, substantially anhydrous, of an alkali metal hydroxide in a high boiling glycol. After treatment of the film in the alkaline glycol solution, the film is rinsed with water, dried and laminated to a thin film of copper foil using a commercial adhesive. The laminates are then cured prior to testing. Example 1 in the '528 patent shows a comparison of the new treatment process with a prior art treatment using an aqueous alkaline solution as disclosed in U.S. Pat. No. 3,361,589. The results indicate only a slight improvement in adhesion (untreated film--1.0 psi) for the aqueous solution (1.25 psi) and a large improvement for the glycol alkaline solution (6.8).
The procedures taught in the above two referenced patents would not be transferable to a process for plating or electroless deposition of metals to plastic substrates. Lamination involves bonding of two films by use of an adhesive. In the '528 patent, the polyimide film is bonded to a copper film using an adhesive to help create the bond. In plating or electroless deposition, the metal is deposited directly on the surface with no intermediate present to improve adhesion.
An object of this invention is to provide an improved process for promoting the adhesion of metal to various engineering thermoplastic substrates without physical modification or degradation of the surface structure. The cohesive integrity of the surface of the substrate is maintained throughout subsequent processing steps. The method provides a means for increasing the adhesion of metallic traces to the surface while maintaining the ability of the surface to be patterned by various techniques.